Your avatar suggests you've already decided...
But I would recommend the 360 because, IMHO (can't stress that enough,) the community is larger and Xbox LIVE is a really versitile system backed by decent support. Yes, you pay for it, unlike PS3's online (which they NOW have a PLUS subscription that costs...) but you really are getting something for your buck. I own both systems and hardly ever turn on my PS3. and every time I do, the system has to take forever to update! Then the updates aren't even significant if they're even noticeable.
Titles-wise, given that pretty much every AAA title is multiplatform, you really can't go wrong with either system for those, but again, IMHO, the 360 has a better line up of exclusives (Halo, Gears of War, etc.) I bought my PS3 for its exclusives only... and after Heavy Rain (which was awesome,) nothing really drew me in or really competes with multiplatform titles.
In the end, I'd say follow the exclusive titles you think you'll like most and see which system it'll defer to. Of course, this being The Escapist, there's bound to be a few 'clever' responses in the vein of "Don't get either one; get a PC."